This application claims the priority of Japanese Patent Application No. 10-92734 fled on Mar. 19, 1998, which is incorporated herein by reference.
The present invention relates to a lithium secondary battery, and more particularly, it relates to improvement of a positive electrode active material used therein for the purpose of providing a lithium secondary battery with an excellent charge-discharge cycle characteristic.
A lithium secondary battery can be improved in its voltage and capacity by appropriately selecting a positive electrode material because there is no need to consider a water decomposition voltage. Therefore, in accordance with recent development of portable and/or cordless electronic equipment, lithium secondary batteries have been regarded as a promising driving power supply for such electronic equipment.
Lithium cobaltite (LiCoO.sub.2) has already been practically used as a positive electrode active material in a lithium secondary battery, and the use results in a battery with a high energy density and a high discharge voltage of the 4-V class. Cobalt, that is, a raw material of lithium cobaltite, is, however, a scarce resource and expensive, and hence, use of lithium nickelate (LiNiO.sub.2) in place of lithium cobaltite is now under examination.
Lithium nickelate has, however, a disadvantage that the capacity of a battery including this material is largely degraded through charge-discharge cycles because its crystal structure is less stable than that of lithium cobaltite.
As a positive electrode active material usable for overcoming this problem, a composite oxide obtained by substituting part of nickel atoms included in lithium nickelate with cobalt atoms and manganese atoms is reported (see Japanese Laid-Open Patent Publication No. 8-37007/1996). However, the stability, during charge-discharge cycles, of the crystal structure of this composite oxide is still not sufficient to be used in place of lithium cobaltite.